Optical communication systems which operate in the wavelength range from 1100 to 1700 nanometers (nm) are of potentially great importance because the dispersion and losses in an optical fiber are typically very low in this wavelength range. Heterojunction devices incorporating binary III-V alloys and solid solutions of these alloys have been found to be particularly useful for this application because their electronic bandgaps occur in this wavelength range and lattice-matched heterojunctions can be obtained by compositional variations. In particular, ternary and quaternary alloys of In, Ga, As and P on an InP substrate have found to be useful materials for both light-emitters and detectors.
Problems which have affected the performance of avalanche photodetectors using these materials include bulk tunneling currents which occur at electric fields of the order of 1.5.times.10.sup.5 V/cm in the ternary and quaternary compounds used for the light-absorptive region, edge breakdown and multiplication of surface leakage currents at the junction periphery. The tunneling has been reduced by locating the P-N junction with its high electric field in a wide bandgap material separated from the light-absorptive region in the narrower bandgap material. This is the so-called SAM (Separated Absorbing and Multiplying) avalanche photodetector structure.
For a SAM avalanche photodetector comprising a P.sup.+ -type substrate, an N-type wide bandgap region, an N-type absorptive region and an N.sup.+ -type cap for contacting purposes, edge breakdown and surface currents are reduced substantially by using sloping sides or surface contouring, thereby forming a mesa structure. This structure has several disadvantages. The region under the contact is multiplying. Illumination through the cap requires that the depletion region extend through the absorptive layer resulting in a high dark current from interface states. With illumination through the substrate the quantum efficiency is reduced by free carrier absorption. For these reasons a mesa structure comprising an N.sup.+ substrate, N-type wide bandgap and absorptive regions and an P.sup.+ -type cap is more desirable. However, the surface contouring then enhances the electric field at the periphery. Thus additional means must be sought to reduce the electric field at the surface so that only avalanche multiplication of photogenerated carriers will occur.